The UPS Debate: A Conversation on High Efficiency, Multi-Mode UPSs

Brad Thrash is a product manager for GE’s Critical Power business with global responsibility for the company’s three-phase UPS product line, including its TLE Series UPS.

BRAD THRASH
GE Critical Power

There are a lot of smart people in the data center power protection market debating the merits, value, risks and rewards of high efficiency uninterruptable power supplies (UPSs). Referred to as multi-mode UPS, or eco-mode, this technology uses smart control logic to switch in milliseconds, as needed, between a premium efficiency mode (multi-mode) and a premium power protection mode (rectifier/inverter double conversion). This improves the energy efficiency of converting alternating current (AC) power to direct current (DC) power by reducing conversion steps required when utility power is within an acceptable tolerance. If there is a power anomaly that affects the load to data center servers and equipment, multi-mode UPSs quickly switch to double conversion mode.

With energy efficiencies topping 98 to 99 percent compared with double conversion technologies operating at 92 to 95 percent levels, these new multi-mode UPS architectures offer significant operating expense (OpEx) savings for data centers. Yet, there’s some industry discussion about both the risks and rewards of energy efficiency versus power reliability and quality. Is the switching, or transfer, technology robust and fast enough to protect the load? Does a four or five percent energy efficiency improvement make a difference in life cycle operating costs? Are there enough multi-mode UPSs deployed at data centers to be statistically significant to make the return on investment (ROI) case?

At GE’s Critical Power business, we’ve been working with energy efficient power solutions for a long time and our eBoost UPS technology, (a multi-mode system), is deployed in many data center locations globally. (See YouTube video which explains this technology.) So we’re a bit biased. Yet the questions about high efficiency UPSs deserve further discussion, so we’ve outlined some of those issues and thoughts below.

Multi-Mode Transfer Speed

The basic premise of any multi-mode UPS depends on balancing and optimizing the time between when stable and clean energy is coming from the utility, and can be efficiently converted to power the load, and when a power anomaly requires the UPS to shift into a double conversion mode. The latter stabilizes the power, but impacts overall power efficiency.

So what’s that optimum switching or transfer time? Some earlier white papers and blogs (1) suggest that anything over eight to 10 milliseconds (ms) is problematic, given that not all data center sensitive equipment (servers, etc.) have tolerances at or above these levels. According to a Green Grid white paper (2) on multi-mode, “if, for example, a UPS has a transfer time of greater than 10 ms and is paired with information technology (IT) equipment that has ride-through capabilities of only 10 ms, the UPS may not be able to support the IT equipment.”

That’s one of the reasons a few companies, including GE, design its multi-mode UPSs with transfer speeds of less than two milliseconds. The technologies that help achieve these speeds are seamless, but represent an intricate combination of power disturbance detection, analysis and control systems.

When eBoost’s responsive monitoring technologies detect any sort of deviation on the main or bypass power path, the inverter is immediately turned on to allow quality power to flow from the double conversion premium protection mode. In the same instant, the static switch on the bypass path from the utility is turned off to block the disturbance from reaching the load. Several patented innovations enable GE’s eBoost technology to accomplish the multi-mode-to-double-conversion mode switching processes in less than two milliseconds.

A variety of disturbance analyzers, some patented by GE, are employed in combination, such as an instantaneous adaptive voltage error detector that monitors subtle changes in amplitude and duration; a root mean square (RMS) voltage error detector that computes the root mean square of all three UPS output voltages for variances; or an output short circuit detector that, after a breaker is tripped, will automatically increase line current to rapidly clear and reset the breaker. A sophisticated transient inverter controller quickly manages the transfer of the load to inverter power and back again to the bypass path.

All these advanced monitoring and control systems work in concert to anticipate and respond to a comprehensive set of possible power conditions, creating transfer switch speeds of less than two milliseconds. This speed helps to maximize the intermittent transfer to double conversion protection, while maintaining higher multi-mode efficiency for the majority of the time when quality utility power is flowing.

Does Multi-Mode Equate to Unprotected Utility Power?

Some data center facility managers fear it’s hard to sell their risk-adverse senior managers on running their data center’s critical operations on multi-mode UPSs which, for some UPS suppliers, is a utility bypass line. At GE, designers incorporate a bypass line reactor that electrically couples with the output filter circuit to provide power line conditioning while the UPS operates in multi-mode (eBoost). This power line conditioning protects against many low level transients of the utility source, thus cutting down the number of possible transfers to the UPS inverter. More than 98 percent of utility power anomalies are voltage transients, so this design feature is quite helpful. The other two percent of power anomalies are brownouts and blackouts, which truly need the double conversion mode and battery backup feature of the UPS system.

Do Percentages Matter?

A third argument concerning multi-mode UPSs we hear in the market is “if our UPS running in double conversion already gets us to 93 percent efficiency, why take a ‘risk’ for a few percentage points in efficiency? Can that extra energy efficiency provide a significant return?”

If we look at a UPS deployment at a typical 10 megawatt (MW) data center realizing just a one percent gain in efficiency, we can see a significant impact over 10 years (see recent Data Center Knowledge Article on UPS TCO)(3). As the chart below shows (Figure One below), while capital expenses (CapEx) are fixed, a total cost of ownership (TCO) evaluation of the OpEx for running an UPS over 10 years creates an operational savings of $1.4 million when energy efficiency improves a single percent − from 93 to 94 percent efficiency. With newer multi-mode UPS technologies that provide up to 96.5 percent efficiency, that savings could jump to almost $3.4 million.

Figure One

Given that data center managers are typically looking to reduce OpEx numbers to cut costs, and hosting companies need to find savings at every level of their operations to remain price competitive, those efficiency percentages matter.

Finding Proof Points

When multi-mode or eco-mode technologies are discussed in trade journals, at conferences and in industry committees, there’s general agreement that multi-mode is the right path for all UPS designs moving forward. While some people still caution that there’s not enough runtime data to make the business use case, that’s not been our experience at GE.

GE conducted two fleet trials during a three-year period from March 2010 to March 2013, which showed some positive efficiency gains in data centers in Atlanta, Ga. and Louisville, Ky. The Atlanta facility, with two GE SG Series 300 kilovolts ampere (kVA) UPSs, operated for more than 25,000 hours with 100 percent UPS reliability, running in eBoost mode 98 percent of the time. The Louisville data center, also operating at 100 percent UPS reliability continuously for more than 21,000 hours, using four GE SG Series 750 kVA UPSs, ran in eBoost mode 95 percent of the time.

More recently, GE helped CoreSpace, a data center, cloud, and hosting provider convert a newly acquired data center in Dallas, Texas, to centralized power management and UPS architecture. They’re using a GE three-phase SG Series 500 kVA UPS system running in eBoost energy-efficiency mode. Since the conversion, these UPSs running in eBoost are providing an overall energy efficiency of 99 percent, with eBoost contributing an annual energy cost savings of $24,800.

As multi-mode technology continues to advance and we see more deployments yield clear and dramatic performance and ROI data, we expect the debate about this efficiency-driving power conversion approach to continue. We invite the industry to join this conversation by adding their comments to journal articles such as this; to share their ideas at industry forums; and to bring forward new data and ideas to advance new approaches for data center power efficiency.

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